1. Field of the Invention
The embodiments of the invention generally relate to a method for determining a reference position of a robot utilized in a semiconductor processing system.
2. Background of the Related Art
Semiconductor substrate processing is typically performed by subjecting a substrate to a plurality of sequential processes to create devices, conductors and insulators on the substrate. These processes are generally performed in a process chamber configured to perform a single step of the production process. In order to efficiently complete the entire sequence of processing steps, a number of process chambers are typically coupled to a central transfer chamber that houses a robot to facilitate transfer of the substrate between the surrounding process chambers. A semiconductor processing platform having this configuration is generally known as a cluster tool, examples of which are the families of PRODUCER®, CENTURA® and ENDURA® processing platforms available from Applied Materials, Inc., of Santa Clara, Calif.
Generally, a cluster tool consists of a central transfer chamber having a robot disposed therein. The transfer chamber is generally surrounded by one or more process chambers. The process chambers are generally utilized to process the substrate, for example, performing various processing steps such as etching, physical vapor deposition, ion implantation, lithography and the like. The transfer chamber is sometimes coupled to a factory interface that houses a plurality of removable cassettes, substrate storage, each of which houses a plurality of substrates. To facilitate transfer between a vacuum environment of the transfer chamber and a generally ambient environment of the factory interface, a load lock chamber is disposed between the transfer chamber and the factory interface.
As line width and feature sizes of devices formed on the substrate have decreased, the positional accuracy of the substrate in the various chambers surrounding the transfer chamber has become paramount to ensure repetitive device fabrication with low defect rates. Moreover, with the increased amount of devices formed on substrates both due to increased device density and larger substrate diameters, the value of each substrate has greatly increased. Accordingly, damage to the substrate or yield loss due to non-conformity because of substrate misalignment is highly undesirable.
A number of strategies have been employed in order to increase the positional accuracy of substrates throughout the processing system. For example, the interfaces are often equipped with sensors that detect substrate misalignment within the substrate storage cassette. See U.S. patent application Ser. No. 09/562,252 filed May 2, 2000 by Chokshi, et al. Positional calibration of robots has become more sophisticated. See U.S. patent application Ser. No. 09/703,061 filed Oct. 30, 2000 by Chokshi, et al. Additionally, methods have been devised to compensate for substrate misplacement on the blade of the robot. See U.S. Pat. No. 5,980,194, issued Nov. 9, 1999 to Freerks, et al., and U.S. Pat. No. 4,944,650, issued Jul. 31, 1990 to T. Matsumoto.
However, these methodologies for increasing the accuracy of the robot generally do not compensate for thermal expansion and contraction experienced by the robot as heat is transferred to the robot from hot wafers and from hot surfaces within the process chambers. As evolving process technology has led to higher operating temperatures for many processes, transfer robots are increasingly exposed to high temperatures. Due to the increase thermal exposure of transfer robots, the increase in robot linkage lengths and reach distances, it has become evident that robotic thermal expansion now substantially contributes to substrate misplacement.
For example, in a process chamber performing physical vapor deposition (PVD), the processing temperature may be as high as 200 degrees Celsius. Additionally, some chemical vapor deposition temperatures reach 400 degrees Celsius. Upon completion of the process within the chamber, a portion (generally the blade and a portion of the linkage) of the robot must enter the chamber and retrieve the hot substrate. While the substrate is held by the robot, thermal energy from the substrate and surrounding area is transferred to the robot linkages. This increase in thermal energy generally causes the linkages to expand, thus shifting the center reference position of the blade without providing feedback to the robot's controller. This causes the blade (and substrate) to be placed in a position different than anticipated by the controller. Cooling the robot linkages creates a similar problem by causing the linkages to shorten as they cool. Thus, the substrate may be mispositioned in another chamber by the robot during subsequent transfers due to the thermal shifting of the center reference position of the blade that may lead to substrate damage and defects in device fabrication.
Moreover, even systems equipped with center finding methods and devices may not account for error introduced by thermal changes to the robot. For example, one substrate center finding method rotates the substrate while a center-find sensor records points along the substrate edges. The substrate center relative to the rotation center is found. With the substrate center position known, the robot is sent to the wafer center position. This technique and others like it find offsets in wafer position but do not find errors in robot positioning. If the robot goes to a position different than an expected because of link length changes, the robot will not be correctly positioned during substrate transfer, which may result in substrate damage or defective processing.
The error may be even more dramatic in devices that perform center finding by collecting wafer edge data while the wafer is on the blade, especially with the robot in a retracted position. This is because the magnitude of the robot position error can be very different in the retracted compared to the extended position.
Additionally, the robot linkages may change length during movement between chambers due to thermal change or a long term affect where the robot temperature changes over many wafers. Thus, the substrate center data determined at one chamber is often not correct by the time the substrate reaches its destination such as a second chamber.
Therefore, there is a need for an improved method for determining a position of a robot.